A light rain began falling at PNC Park at around 4:30 p.m. and the National Weather Service issued a severe thunderstorm warning for Pittsburgh and surrounding areas until 6 p.m. ET.
The NWS forecast for Monday night called for scattered showers for the area. The Derby is scheduled to begin at 8 p.m. ET.
"The Pirates and Major League Baseball have been in contact with local weather experts," said Patrick Courtney, MLB vice president, public relations. "We'll keep monitoring [the situation] and make a decision based on conditions and what information we have from [meteorologists]."
MLB will wait as long as is practical to get the Derby in although there is no set rule of thumb on how long to wait in the event of a rain delay. The decision to proceed or postpone is the responsibility of MLB.
The Home Run Derby has been rained out before, most recently in 1988 at Cincinnati's Riverfront Stadium. Likewise, if this year's Derby is rained out it will not be rescheduled.
Jim Molony is a writer for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.